CD4+ T lymphocytes play a central role in the mammalian immune response to foreign antigens by producing lymphokines that are essential for the regulation of cell-mediated and humoral effector mechanisms. CD4+ T cells require at least two signals to produce lymphokines maximally. One signal is provided by T cell antigen receptor (TCR) occupancy by antigenic peptides bound to major histocompatibility complex molecules on the APC surface, the other by the interaction of an as yet unidentified receptor(s) on the T cell with a complementary ligand(s) on the APC. For certain T cell clones, TCR occupancy in the absence of the costimulatory signal not only fails to induce lymphokine production but instead results in anergy, i.e., the T cells are unable to produce lymphokines when challenged with both signals. The broad objective of this proposal is to identify costimulatory molecules and determine how they regulate lymphokine production and anergy. Specifically: (a) cell cycle inhibitors will be used to test whether TCR occupancy results in anergy because T cells do not receive costimulatory signals or because they do not undergo cell division; (b) heterokaryons produced from the fusion of anergic and normal T cell clones will be used to determine if anergy is maintained by an inhibitory intracellular factor; (c) the mechanism and specificity of T cell unresponsiveness induced in vivo in a graft-versus-host disease system will be studied; and (d) based on striking correlations between the biological effects of costimulatory APC and anti-CD28 antibodies, human CD28 will be evaluated as a potential T cell costimulation receptor and its ligand B7 as a potential costimulatory molecule on the APC. Successful completion of these experiments will not only broaden our knowledge of the molecules involved in T cell activation but should also lay the foundation for new immunomodulatory strategies aimed at augmenting (e.g. in tumor immunity) or inhibiting (e.g. in transplantation or autoimmunity) costimulatory receptor/ligand interactions.